The present invention relates to the transmission of data over the physical layer in a multilayer data communication scheme, and is particularly applicable to the transmission of upstream messages in a hybrid fiber coaxial transmission system. The invention is useful, for example, in communicating messages from a subscriber terminal to a cable television headend for purposes of providing, e.g., Internet access over a cable television network.
In the past, cable television systems have utilized coaxial cable to carry signals from a headend to individual subscriber terminals. With the advent of new digital television services as well as the desire to carry data to and from subscriber terminals, the increased bandwidth provided by fiber optic cable has become attractive. Implementation of optical fiber networks all the way from the headend to individual subscriber terminals is not currently a practical alternative due to the high cost of building an optical fiber plant in which fiber is run all the way to individual homes. As a compromise, hybrid fiber coax (HFC) plants are being implemented.
In HFC systems, fiber is run from the headend to neighborhood hubs. Existing coaxial cable is then coupled to receive the signals from the optical fiber, for distribution to individual homes.
Various interactive services to be provided by digital transmission systems require a bidirectional link between the headend and individual subscriber terminals. One way to provide a return path from the subscriber terminals back to the headend is to rely on existing telephone lines. However, it would clearly be advantageous to provide bidirectional communication over the same plant, such as an HFC plant, in which the downstream television signals are provided to the subscriber terminals. In order to accomplish this, proposals have been made to develop robust upstream communication systems over HFC plants. For example, single carrier frequency and time division multiple access (F/TDMA) have been proposed to provide a low risk, high capacity approach which offers suitable characteristics for upstream modulation over an HFC system.
In order to implement a practical upstream channel, it would be advantageous to provide various choices to a system operator to accommodate different needs. For example, it would be advantageous to offer a trade-off between data throughput (i.e., bandwidth efficiency), error rate performance (i.e., robustness) and latency. In offering such flexibility, it would be particularly advantageous to offer system operators a frequency agile carrier, choices between different modulation techniques, such as quadrature phase shift keyed (QPSK) and quadrature amplitude modulation (QAM), multiple symbol rates, flexible forward error control (FEC) coding, and flexible frame and preamble structure for the data packets transported over the communication channel. Such choices would allow many opportunities for different system operators to find satisfactory sets of operational modes to serve the needs of subscribers economically and efficiently.
The present invention provides a flexible communication scheme enjoying the aforementioned and other advantages.